The major objective of this grant is to define the relative contributions of polyreactive nd monoreactive antibodies against platelets in HIV-1 associated thrombocytopenia (HIV-ITP). The central hypothesis is that HIV-ITP is a result of both polyreactive and monoreactive platelet antibodies. The applicant has used an antibody-IgG phage combinatorial library to isolate monoclonal Fab fragments from the bone marrow of 2 HIV-ITP patients which react with human platelets. He plans to: 1) Test these Ab's in 2 mouse models: a Balb/c mouse known to react with human anti-GPIIIa Ab by developing thrombocytopenia, and a SCID mouse engrafted with human platelets where the human Ag's share no homology; 2) Generate human IgG1 Ab's by attaching the constant region to the Fab Ab's-and determine their in vivo effect; 3) Isolate a panel of platelet-associated Ab's by panning vs platelets and their products obtained from normal and HIV-ITP patients; 4) Define the platelet-associated Ag's recognized by these filamentous Fab Ab's by classical means as well as by employing a phage directed platelet cDNA library to produce the Ag-and determine the relative contributions of these Ab's against patient serum Ab's with in vitro competition assays as well as with in vivo effect of these Ab's. He will also look for molecular mimicry with foreign Ag's.